Cast Iron & Galvanized Pipe Rusted Connection

Could somebody offer a suggestion on removing a galvanized nipple to a rusted cast iron P trap for a bath tub. Drain system is under a concrete slab. I intend to replace the tub due to corrosion & rust.Is it also possible that the threads on the trap are gone due to corrosion that makes reconnection w/
new pipe impossible?

How large a hole will he have to make in the slab to get, and turn, a 24" wrench? And what are the chances the trap will snap if he uses that much torque? Cut the nipple out with a SawZall. If you cannot figure out how to do it, call a plumber who does. Your thread in the trap is probably still good. IF not the nipple would have just pulled out or turned easily.

HJ, JIMBO
I appreciate your help & thanks.Will pre heating the connection w/ propane
torch help loosen fittings & where could I get the PB blaster penetrating oil
since this is my first time hearing this type of oil. I know of WD40.
Thanks again.